The invention relates generally to locating the position of a mobile communication unit in a wireless communication network and, more particularly, to making downlink observed time difference measurements.
The ability to locate the position of a mobile communication unit operating in a wireless communication system (for example, a cellular communication system) provides many well known advantages. Exemplary uses of such position location capability include security applications, emergency response applications, and travel guidance applications. Several known techniques for providing position location involve the measurement of certain characteristics of communication signals, such as the time of arrival (TOA), the round trip delay, or the angle of arrival of a communication signal. Some of these techniques can be further divided into uplink or downlink approaches. In the uplink category, a base transceiver station (BTS) or other receiver performs the measurements on communication signals originating at a mobile communication unit (or mobile station). In downlink approaches, the mobile station performs the measurements on signals originating at base transceiver stations or other transmitters.
One example of a downlink technique for locating the position of a mobile station is the observed time difference (OTD) technique. This technique will now be described with respect to the Global System for Mobile Communication (GSM), which is exemplary of a cellular communication system in which downlink observed time difference techniques are applicable. The OTD technique is implemented, for example, by having the mobile station measure the time difference between arrival times of selected radio signals transmitted from different base transceiver stations. Assuming the geometry shown in FIG. 1, and further assuming that two signals are transmitted simultaneously from the base transceiver stations BTS1 and BTS2, and letting T1 and T2 denote the times of arrival of the respective signals at the mobile station, then the observed time difference OTD is given by the following equation:
T1-T2=(d1-d2)/c,xe2x80x83xe2x80x83(Eq. 1)
where d1 and d2 are the respective distances from BTS1 and BTS2 to the mobile station. The locations of BTS1 and BTS2 are known, and the possible locations of the mobile station are described by the hyperbola 15 shown in FIG. 1. By combining measurements from at least three base transceiver stations, a position estimate for the mobile station can be obtained.
Most conventional cellular communication systems (including GSM systems) are asynchronous, that is, each base transceiver station uses its own internal clock reference to generate its frame and time slot structure. Therefore, the frame structures of the different base transceiver stations will tend to drift in time relative to one another, because clocks are not perfectly stable. As a consequence, an OTD measurement is not really meaningful for locating the position of a mobile station unless the differences in timing between the base transceiver stations being used is known. This difference, often referred to as the real time difference or RTD, represents the actual difference in absolute time between the transmission of respective signals (e.g., respective synchronization bursts in GSM) from respective base transceiver stations, which signals would be transmitted simultaneously if the frame structures of the base transceiver stations were perfectly synchronized.
Among several possible approaches to determine the real time difference RTD between base transceiver stations, two conventional examples are: absolute time stamping in the respective base transceiver stations; and use of stationary reference mobiles located in known positions. In the latter example, the reference mobile measures downlink signals sent from various base transceiver stations. Because the respective distances between the various base transceiver stations and the stationary reference mobile station are known, the expected time difference in arrival times of the respective signals from the base transceiver stations can be easily calculated. The real time difference RTD between base transceiver stations is simply the difference between the expected time difference of arrival and the observed time difference of arrival actually observed at the reference mobile station. The reference mobile station can periodically make the downlink time of arrival measurements and report them to a mobile location node in the network so that the network can maintain an updated record of the RTDs.
The techniques underlying known OTD methods are very similar to procedures used conventionally by mobile stations to synchronize to a serving base transceiver station and make measurements on a number of neighboring base transceiver stations as instructed by the serving cell (as in mobile assisted hand-off operations). The mobile station needs to know which base transceiver stations are to be monitored for OTD measurements. This information can typically be provided in conventional system information messages broadcasted in the cell, for example on a GSM cell""s BCCH (broadcast control channels) frequency. This system information typically includes a list of frequencies of neighboring cells which are to be measured. The mobile station scans the designated frequencies to detect a frequency correction burst, which is an easily identifiable burst that appears approximately every 50 ms in GSM.
After successful detection of a frequency correction burst, the mobile station knows that in GSM the next frame will contain a synchronization burst SB. The synchronization burst SB contains the Base Station Identity Code (BSIC) and information indicative of the frame number of the current frame in which the burst SB is occurring. The mobile station measures the time of arrival of the synchronization burst SB at the mobile station relative to the timing of mobile station""s own serving cell. Since now the mobile station knows the frame structure of the neighboring base transceiver station relative to its own serving base transceiver station timing, it is possible to repeat the time of arrival measurements to improve accuracy. This procedure is repeated until all frequencies (i.e., all BTSs) on the list have been measured. The observed time difference values recorded by the mobile station are then transferred to a mobile station location node in the cellular system, which node performs the position determination based on the observed time difference values, the real time difference values and the geographic locations of the base transceiver stations.
Because the mobile station does not know when the frequency correction burst (and thus the following synchronization burst SB) will appear, the brute force method described above, namely monitoring for the frequency correction burst, must be used.
The time required to capture a synchronization burst will depend on the measurement mode. OTD measurements can be made, for example, when call setup is being performed on a GSM SDCCH (Stand-alone Dedicated Control Channel), or during idle frames when the mobile station is in call mode, or during speech interrupt. For example, if the mobile station makes the measurements in call mode, then the mobile station can only make measurements during idle frames, which conventionally occur in GSM systems every 120 ms. The probability that a particular synchronization burst will appear within the idle frame is approximately 1 in 10, because the synchronization burst conventionally occurs once every ten frames in GSM. Accordingly, on average, 5 idle frames will be needed, meaning 0.6 seconds per base transceiver station. Thus, if it is desired to measure at least 6 neighboring base transceiver stations, an average measurement time of 3 or 4 seconds will be required, which may be prohibitively long in many applications.
The mobile station is guaranteed to have measured the synchronization burst SB if the mobile station captures and stores all signals (for example, all signals on the BTS""s BCCH frequency in GSM) for 10 consecutive frames. However, providing the mobile station with the memory and computational capacity to capture (and thereafter process) all signal information in 10 consecutive frames is disadvantageously complex.
Moreover, in areas such as urban areas characterized by high interference levels, and in rural areas with large distances between base transceiver stations, the probability of detecting the synchronization burst SB may be unacceptably low, because the signals will typically be characterized by low signal-to-noise ratios.
Due also to the low signal-to-noise ratio, it is typically very difficult to decode the BSIC in the synchronization burst SB. The probability of taking ghost spikes instead of a synchronization burst SB is therefore disadvantageously increased in instances of low signal-to-noise ratio.
For locating a mobile station operating in a network using a Code Division Multiple Access (CDMA) air interface, one known downlink OTD approach, which has been proposed for standardization, uses some conventional cell-search signals provided in the CDMA network. This known downlink OTD approach is also referred to hereinafter as the xe2x80x9cproposedxe2x80x9d approach or technique. Examples of conventional mobile communication systems that employ a CDMA air interface include so-called Wideband CDMA (WCDMA) systems such as the ETSI Universal Mobile Telecommunication System (UMTS) and the ITU""s IMT-2000 System. In such systems, the proposed downlink OTD positioning technique is performed by the mobile station during predetermined idle periods wherein the mobile station""s serving base transceiver station ceases all transmission in order to enhance the mobile station""s ability to detect signals transmitted by neighboring base transceiver stations. Certain signals conventionally provided for cell-searching in the aforementioned CDMA systems, namely a first search code (FSC) and a second search code (SSC), are also used in performing downlink OTD positioning.
During the idle period(s) of its serving base transceiver station, a mobile station uses a matched filter that is matched to the first search code FSC, just as is done in conventional cell-searching. The FSC is conventionally transmitted by all base transceiver stations in CDMA networks such as mentioned above. The FSC is 256 chips long and is transmitted by each base transceiver station once every time slot, that is, one tenth of the time (each time slot is 2,560 chips long). Each ray of each base transceiver station within the mobile station""s hearable range results in a peak in the signal output from the matched filter. In the conventional peak detection process, the results from several time slots are typically combined non-coherently to improve the peak detection. In conventional cell-searching, the mobile station typically chooses the strongest detected peak. However, in the proposed downlink OTD positioning technique, the time of arrival (TOA) of each detected peak is measured by the mobile station using conventional time of arrival measurement techniques, so the observed time differences (OTDs) between the times of arrival of the respective peaks can be calculated.
Each base transceiver station operating in the aforementioned CDMA networks also conventionally transmits an associated second search code (SSC), which includes a set of 16 codes arranged and transmitted in a certain order. The 16 codes are transmitted sequentially, one code per time slot, and each of the 16 codes is transmitted simultaneously with the FSC transmitted in that time slot. The exemplary conventional CDMA systems mentioned above have 16 time slots per frame, so the entire SSC pattern, including all 16 codes, is repeated once every frame. The SSC pattern, with its 16 codes arranged in a certain order, specifies, from among a plurality of possible code groups, a single code group associated with the base transceiver station. Each code group includes a plurality of CDMA spreading codes, and each base transceiver station uses one of the spreading codes from its associated code group.
For each base transceiver station within hearable range, a mobile station performing the proposed downlink OTD positioning technique correlates the temporal location of that base transceiver station""s FSC peak with the 16 codes of its SSC pattern, just as is done in conventional cell-searching. This correlating process typically uses non-coherent combining. If the peak is successfully correlated with an SSC pattern, then this correlation result indicates the code group associated with the base transceiver station that produced the FSC peak.
The FSC peak timing (i.e., the measured TOAs and/or OTDs) and the code group for each detected base transceiver station can then be reported to a mobile location node in the network, along with power and quality measurements made during the FSC peak detection process and during the FSC-SSC correlation process.
The mobile location node already knows the RTDs among the base transceiver stations (conventionally obtained from, e.g., absolute time stamping in the base transceiver stations, or a stationary reference mobile station), and thus knows, within a range of uncertainty due to the unknown location of the mobile station, when the mobile station should have received the FSC peak from any given base transceiver station. Using this known RTD information, in combination with the aforementioned peak timing, power and quality information received from the mobile station, the mobile location node can identify the base transceiver station corresponding to each FSC peak.
For example, if the location of the mobile station is known within a 4.5 kilometer range of uncertainty, this range corresponds to 64 chips. If the frame structure timing of one candidate base transceiver station differs from that of another candidate base transceiver station in the same code group by more than the 64 chip uncertainty, then the correct one of those base transceiver stations can always be determined. Assuming that the frame structure timing of each base transceiver station is random, the probability that any two base transceiver stations will have a frame structure timing difference therebetween (that is, a real time difference RTD) of 64 chips or less is 64/40,960, because each frame includes 40,960 chips (16 timeslotsxc3x972560 chips/timeslot). Thus, the probability that a peak produced by one base transceiver station can be distinguished from a peak produced by another base transceiver station of the same code group is 99.8% (1-64/40,960). The other 0.2% of situations can be handled by more advanced schemes, for instance by using power measurements and by choosing the base transceiver station that gives the best fit in a conventional location-determining cost-function.
Once each FSC peak has been matched to its corresponding base transceiver station, the TOA and/or OTD information can be used, in combination with the known RTD information and the known geographical locations of the base transceiver stations, to determine the geographical position of the mobile station.
The proposed downlink OTD positioning technique has the following exemplary disadvantages. Because the timing of the neighboring (non-serving) base transceiver stations is completely unknown to the mobile station when it begins the downlink OTD process, the mobile station must perform the FSC-SSC correlation processing for the entirety of its base transceiver station""s idle period(s). Thus, the matched filter used in detecting the FSC peak must disadvantageously operate for the entire length of each idle period. Also, because the codes in the SSC pattern are different in each time slot, the mobile station must correlate with several SSCs, and then save the results for non-coherent combining. This disadvantageously requires additional computation capability and additional memory.
Because the FSC-SSC correlation processing must sequentially follow the FSC peak detection, the acquisition time in the proposed downlink OTD approach can be disadvantageously long. Also, urban areas characterized by high interference levels, and rural areas with large distances between base transceiver stations, can make it difficult, and sometimes impossible, to detect the FSC and the SSC with sufficient probability.
Another problem is that the codes associated with different base transceiver stations have quite high cross-correlations, because the FSC codes are all identical and because the 16 codes of each SSC pattern represent a subset produced from a set of 17 unique codes. These high cross-correlations do not die out with increased numbers of combined correlations, because the same codes are repeated in every frame. This disadvantageously increases the probability that the mobile station may correlate a given FSC peak to the wrong SSC pattern, especially if the FSC from a strong base transceiver station arrives temporally close to the FSC from a weaker base transceiver station.
It is desirable in view of the foregoing to improve the mobile station""s ability to detect downlink signals used in known downlink observed time difference approaches.
The present invention attempts to overcome the aforementioned disadvantages of known downlink observed time difference approaches by providing for improved sensitivity in detecting the downlink communication signals used for making observed time difference measurements at mobile stations.